In a study of 185 women ages 40 to 50, 22% of those who had a mammogram in the year prior to diagnosis had a mastectomy, compared with 52% who did not have a mammogram in the previous year (p = 0.02).

In addition, women who had been screened in the year prior to diagnosis had smaller tumors: 17.8 mm, on average, compared with 24 mm for those who'd had mammography more than a year ago and 28.8 mm for those who never underwent screening.

They also had less likelihood of multifocal malignancy -- 12% versus 34% for those who were never screened (p = 0.003) -- and they were less likely to have high-grade disease, at 31% versus 46%, respectively (p = 0.03).

The odds of mastectomy also correlated with the size of the tumor (p = 0.04), according to Nicholas Perry, director of the London Breast Institute at Princess Grace Hospital in London.

"Our study delivers strong clinical evidence to support annual screening of women from age 40," he said.

The researchers reviewed clinical and imaging data on women who were seen at their institute between 2003 and 2009. Only 26% had mammograms prior to their diagnosis.

Of the 48 women who had ever had a mammogram, 18 had one within the year prior to diagnosis, 15 had the exam one to two years prior, and 15 had a mammogram more than two years earlier.

Perry said that the U.K. does not recommend routine breast cancer screening for women younger than 50.

"The results are striking but should not be surprising," he said, adding that he doubted they would result in a change in recommendations.

A total of 37,000 U.S. women and 7,000 British women ages 40 to 50 are diagnosed with breast cancer each year, according to Perry. Based on these rates, yearly mammograms would spare 10,000 U.S. women and 2,000 U.K. women from mastectomy annually, he said.

When asked if the annual breast screens improved overall survival, he said longer-term studies are needed.